## A Model For Halo Formation With Axion Mixed Dark Matter    [PDF]

David J. E. Marsh, Joe Silk
There are several issues to do with dwarf galaxy predictions in the standard $\Lambda$CDM cosmology that have suscitated much recent debate about the possible modification of the nature of dark matter as providing a solution. We explore a novel solution involving ultra-light axions that can potentially resolve the missing satellites problem, the cusp-core problem, and the `too big to fail' problem. We discuss approximations to non-linear structure formation in dark matter models containing a component of ultra-light axions across four orders of magnitude in mass, $10^{-24}\lesssim m_a \lesssim 10^{-20}$, a range too heavy to be well constrained by linear cosmological probes such as the CMB and matter power spectrum, and too light for other astrophysical or terrestrial axion searches. We find that an axion of mass $m_a\approx 10^{-21}\text{eV}$ contributing $\Omega_a/\Omega_d \gtrsim 0.85$ of the total dark matter can introduce a significant kpc scale core in a typical Milky Way satellite galaxy in sharp contrast to a thermal relic with a transfer function cut off at the same scale, while still allowing such galaxies to form in significant number. Our model simultaneously allows production of enough high redshift galaxies to allow reconciliation with observational constraints, and also reduces the maximum circular velocities of massive dwarfs so that baryonic feedback may more plausibly resolve the predicted overproduction of massive MWG dwarf satellites.
View original: http://arxiv.org/abs/1307.1705